Add java examples and fix uncovered underscore case in hex literals

Removed redundant test cases

https://docs.oracle.com/javase/8/docs/technotes/guides/language/underscores-literals.html

Author: Jentsch

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -508,6 +508,8 @@ object Scanners {
             if (ch == 'x' || ch == 'X') {
               nextChar()
               base = 16
+              if (isNumberSeparator(ch))
+                errorButContinue("leading separator is not allowed", offset + 2)
             } else {
               /**
                * What should leading 0 be in the future? It is potentially dangerous

tests/neg/t6124.check

@@ -1,22 +1,34 @@
-<304..304> in t6124.scala
-double precision floating point number too small
-<273..273> in t6124.scala
+[490..493] in t6124.scala
+Not found: _52
+[404..406..412] in t6124.scala
+value _1415F is not a member of Int
+<594..594> in t6124.scala
 trailing separator is not allowed
-<235..235> in t6124.scala
-Invalid literal number
-<240..240> in t6124.scala
+<567..567> in t6124.scala
+leading separator is not allowed
+<540..540> in t6124.scala
 trailing separator is not allowed
-<213..213> in t6124.scala
+<516..516> in t6124.scala
 trailing separator is not allowed
-<187..187> in t6124.scala
+<467..467> in t6124.scala
 trailing separator is not allowed
-<160..160> in t6124.scala
+<375..375> in t6124.scala
+trailing separator is not allowed
+<228..228> in t6124.scala
+double precision floating point number too small
+<197..197> in t6124.scala
 trailing separator is not allowed
-<125..125> in t6124.scala
+<159..159> in t6124.scala
 Invalid literal number
-<101..101> in t6124.scala
+<164..164> in t6124.scala
 trailing separator is not allowed
-<67..67> in t6124.scala
+<137..137> in t6124.scala
 trailing separator is not allowed
-<33..33> in t6124.scala
+<111..111> in t6124.scala
+trailing separator is not allowed
+<84..84> in t6124.scala
+trailing separator is not allowed
+<49..49> in t6124.scala
+Invalid literal number
+<25..25> in t6124.scala
 trailing separator is not allowed

tests/neg/t6124.scala

@@ -1,8 +1,5 @@
 
 trait T {
-  def i: Int = 123_456_ // error
-  def j: Long = 123_456_L * 1000 // error
-
   def f = 3_14_E-2 // error
   def e = 3_14E-_2 // error
   def d = 3_14E-2_ // error
@@ -14,5 +11,19 @@ trait T {
 
   def tooSmall = 1.0E-325 // error
 
+  // Examples from
+  // https://docs.oracle.com/javase/8/docs/technotes/guides/language/underscores-literals.html
+
+  val pi1 = 3_.1415F // error
+  val pi2 = 3._1415F // error
+  val socialSecurityNumber1
+    = 999_99_9999_L // error
+  val x1 = _52 // error
+  val x3 = 52_ // error
+
+  val x5 = 0_x52 // error
+  val x6 = 0x_52 // error
+  val x8 = 0x52_ // error
+
   def z = 0
 }

tests/pos/t6124.scala

@@ -8,4 +8,11 @@ trait T {
   def d = 3_14E-2_1
 
   def z = 0
+
+
+  // Examples from
+  // https://docs.oracle.com/javase/8/docs/technotes/guides/language/underscores-literals.html
+  val x2 = 5_2;              // OK (decimal literal)
+  val x4 = 5_______2;        // OK (decimal literal)
+  val x7 = 0x5_2;            // OK (hexadecimal literal)
 }